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Appl Environ Microbiol. 1982 April; 43(4): 823-828

Nitrogen Fixation (Acetylene Reduction) Associated with Duckweed (Lemnaceae) Mats

Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas 77843

ABSTRACT

Duckweed (Lemnaceae) mats in Texas and Florida were investigated, using the acetylene reduction assay, to determine whether nitrogen fixation occurred in these floating aquatic macrophyte communities. N₂-fixing microorganisms were enumerated by plating or most-probable-number techniques, using appropriate N-free media. Results of the investigations indicated that substantial N₂-fixation (C₂H₂) was associated with duckweed mats in Texas and Florida. Acetylene reduction values ranged from 1 to 18 µmol of C₂H₄ g (dry weight)^–1 day^–1 for samples incubated aerobically in light. Dark N₂ fixation was always two- to fivefold lower. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (7 to 10 µM) reduced acetylene reduction to levels intermediate between light and dark incubation. Acetylene reduction was generally greatest for samples incubated anaerobically in the light. It was estimated that 15 to 20% of the N requirement of the duckweed could be supplied through biological nitrogen fixation. N₂-fixing heterotrophic bacteria (10⁵ cells g [wet weight]^–1 and cyanobacteria (10⁵ propagules g [wet weight]^–1 were associated with the duckweed mats. Azotobacter sp. was not detected in these investigations. One diazotrophic isolate was classified as Klebsiella.